An optical or radio-frequency (RF) source is commonly referred to as a frequency comb when it is represented by multiple tones that are equidistantly spaced in frequency domain. Various mechanisms can be used to generate a frequency comb. These are often classified with respect to the spectral comb bandwidth (frequency range over which frequency tones are generated), frequency stability, the spectral tone purity or signal-to-noise ratio (SNR) between the frequency tone and the noise measured within a specific bandwidth in its immediate spectral vicinity, the coherence and its power properties. Frequency combs can be used to establish a spectral reference that can be used to relate the position of spectral absorption or emission in precision ranging, spectroscopy or sensing applications. Consequently, the coherence properties of frequency comb are of particular importance and are defined by combined phase and amplitude noise present at each frequency tone that comprises the comb. In a variety of optical sources that are well stabilized in power, the phase noise dominates over the amplitude noise. Consequently, to preserve the clarity of the invention, we refer to phase noise only; this abbreviation is not meant to exclude the effect that the embodiments will have, in practice both noise mechanisms.
Phase noise manifests itself by broadening the spectral width of the laser (source) spectral width, commonly referred to as a linewidth. This effect is generally seen as an impairment, as broadening of the source linewidth decreases its coherency. Not only does the coherency decrease diminish the utility of optical radiation in applications such as communication and sensing, it also reduces the utility of the source for the purpose of frequency referencing. In practical terms, an ideal frequency reference would have negligible linewidth, allowing one to relate to any other frequency with arbitrary accuracy. In contrast, the physical source possesses a finite linewidth, inherently reducing the accuracy with which frequency gauging can be accomplished. Consequently, it is of large practical interest to reduce the spectral linewidth of the source; this motivation is particularly strong in the case of spectral comb generation, as each frequency tone included in the comb possesses a finite spectral linewidth.
The spectral reference can be established either locally, when the bandwidth of the frequency comb is smaller than a spectral octave, or globally, when the bandwidth of the frequency comb is equal to or exceeds a spectral octave. Both type of frequency combs can be used in metrology, spectroscopy, clock distribution, physical ranging and waveform synthesis, among other applications. To be practically useful, a device or means for frequency comb generation should be power efficient, possess sufficient spectral bandwidth, be characterized by a power equalized spectrum across the operational bandwidth and have high degree of coherency. The latter requirement can be described in terms of the spectral linewidth of frequency tones that constitute the frequency comb. In a coherent frequency comb generation process, the spectral linewidth of frequency tones should not increase with respect to a specific value commonly established at the input of the generating device. Of equal importance, the constant linewidth should be maintained across the entire comb range, regardless of the specific tone frequency position.
Common techniques for frequency comb generation include, in direct or indirect form, the use of optical or RF cavities to establish a frequency reference. Frequency comb generation using mode-locked (MLL) lasers is particularly widespread, and can be used in conjunction with nonlinear processes outside of the MLL cavity. An MLL source inherently represents the frequency comb: pulsed temporal output, when observed in spectral domain, correspond to a frequency comb whose bandwidth is directly defined by a gain width of the laser material, with the rest of parameters dictated by the specifics of the physical mode locking mechanism. In the temporal domain, the separation between adjacent optical pulses of an MLL output is referred to as the repetition rate; in the spectral domain, an inverse of the repetition rate defines the frequency pitch (separation between adjacent spectral peaks) of the frequency comb. An MLL is often used to seed the nonlinear process in order to enhance the bandwidth or other performance parameters of the frequency comb. When coupled with various feedback mechanisms, this approach has led to the demonstration of wideband and low-noise devices used in wave-forming, ranging and spectroscopy.
The use of an MLL source for frequency comb generation necessarily introduces performance limitations. The most severe limit is imposed by requirement placed on the MLL cavity stability. In the case when the MLL cavity is not absolutely stabilized, its output is characterized by temporal and spectral uncertainty (fluctuations). In the case when a nonlinear process is used to expand or enhance the MLL response to create a frequency comb, these fluctuations are further amplified, thus degrading the accuracy and overall performance of the frequency comb source. While many techniques for MLL stabilization were reported and developed in the past, the fundamental limit is established by physical coupling between the frequency pitch (repetition rate) and the cavity physical size. A higher repetition rate (higher frequency pitch) requires a shorter physical cavity in either the optical or the RF domain. Consequently, the tolerance required to control such a cavity length decreases until it reaches a physical scale that cannot be physically realized.
In an alternative to the conventional approach, frequency comb synthesis can be accomplished without an MLL seed. Instead, a continuous-wave (CW) source can be used to seed the frequency comb generation process. As cavity stability of the CW source exceeds that of the MLL source, the inherent limits associated with an MLL or any other pulsed source can be circumvented.
Despite the availability of some optical sources providing frequency combs, there is a need in the art for methods and systems for fiber-based amplifiers with repeatable output pulse characteristics independent of the pulse repetition frequency.